wonderful work of 白蛟会. Not sure why it still wasn't uploaded here so I decided to do the honours.

If you enjoyed it please support the author and buy it.
You can buy it here:
https://www.dmm.co.jp/dc/doujin/-/detail/=/cid=d_175185/?dmmref=ListRanking&i3_ref=list&i3_ord=1

BTW can anyone recommend a newbie like me a good way of jpg compression?
Author of this work went crazy and original raw had like 1,5 GB. every single page had at least 10MB.
I used Caesium to compress it but it seems like 1MB-2MB for page is still a tad excessive.
Is there some freeware that would allow me to compress this to 0,5MB per page without any noticeable loss of quality?

Theoretically you are not supposed to upload a compressed version but the original version. The site supports JPEG files up to 20MB, I see no reason to compress in this case. Also if the original files were 10MB per page and now are 1.5MB, you haven't just compressed a little bit but probably too much. I invite you to zoom in on the original file and on one of your compressed files, you will probably see a difference in the details (presence of artifacts on the compressed version). If someone else buys this doujin and decides to upload it, you are eligible for an expunge as it is not the original version.

Base +29, tobeunforgiven2 +21

Maybe convert the grayscale pages to PNG? I think it's a better choice if you don't want the loss of quality while keeping the file size smaller.

Base +8, sgc_geh +9, StatisticallyNP +9, Argh_ +6

https://www.pixiv.net/artworks/80965238

Base +53, elgringo +64

最后还是交代了一下开头的花啊……

Base +5

贤者之石 黑莲花形态

Base +7

For grayscale files with low artifacts, properly leveled scans or digital releases 8 bit PNG with max compression is the best option. Note that unlike JPG, PNG does not lower the quality of the picture.
For color files PNG will almost always be much larger than JPG.

xnviewMP makes all of this really easy:
/g/1505417/a3e63ff2c6/

Base +7